A light emitting diode is a p-n junction device that converts electrical energy into optical radiation. In particular, under proper forward-biased conditions LED's emit external spontaneous radiation in the ultraviolet, visible, and infra-red regions of the electromagnetic spectrum.
As known to those familiar with the visible and near-visible portions of the electromagnetic spectrum and their characteristics, shorter wavelengths of light (such as blue and ultraviolet) represent higher frequency, higher energy transitions, and longer wavelengths (such as red and infra-red) represent lower frequency, lower energy transitions.
Thus, with respect to light emitting diodes, the particular portion of the spectrum in which they emit—i.e., their color—is based upon the energy of the transitions that create the emissions. In turn, the energy of the transitions is determined to a great extent by the bandgap of the particular material. Thus, in order for a light emitting diode to emit in the blue or ultraviolet portions of the spectrum, the bandgap of the semiconductor material must be large enough (wide enough) to support a transition with sufficient energy to produce blue or ultraviolet light
Accordingly, the candidate materials for light emitting diodes in the blue and ultraviolet regions of the spectrum are limited to certain wide bandgap materials such as diamond, silicon carbide (SiC) and Group III nitrides; e.g., binary, ternary and quaternary nitrides formed from the Group III elements of the periodic table such as gallium nitride (GaN), indium gallium nitride (InGaN), and aluminum gallium nitride (AlGaN).
Recent development work in the field of blue LEDs has focused more closely on the Group III nitrides because of their wide bandgaps and their characteristics as direct, rather than indirect, transition materials. As is well understood by those of ordinary skill in this art, a direct band gap material tends to offer higher efficiency because its energy conversion is predominantly in the form of light (a photon) rather than partially as light and partially as vibrational energy (a phonon).
A more extensive discussion of the structure, quantum mechanics, and operation of LEDs and other photonic devices is set forth in Sze, Physics of Semiconductor Materials, 2d Edition (1981, John Wiley & Sons, Inc), and its companion, Sze, Modern Semiconductor Device Physics (1998, John Wiley & Sons, Inc ). These principles are generally well understood in this art and will not be repeated herein other than as necessary to explain and support the claimed invention.
In a basic sense, a light emitting diode generally includes two layers of opposite conductivity type material which together form a p-n junction. These materials are typically in the form of epitaxial layers on a substrate. Most desirably an ohmic contact is made to the substrate and to the top epitaxial layer to form a “vertical” device for optimum efficiency in packaging.
In this regard, an LED is often packaged for end use in the form of an LED lamp. A typical LED lamp includes an LED chip (or “die”, the term “chip” often being used to describe an integrated circuit rather than an LED) and a plastic (or sometimes glass) lens. For some LEDs the lens is colored to serve as an optical filter and to enhance contrast, but for blue LEDs the lens is preferably colorless so as to avoid interference with the desired blue emission. Typical lamp configurations are well known to those of ordinary skill in this art and are set forth for example, in Sze, Physics of Semiconductor Materials, supra at pages 697–700. Typically, once an LED chip is packaged as a lamp, it can be used for a variety of applications such as indicators and alpha numeric displays.
There are some specific considerations, however, that apply to certain types of devices. For example, Group III nitride devices are typically formed on either sapphire or silicon carbide substrates. Silicon carbide substrates are preferred in many circumstances because silicon carbide (SiC) can be conductively doped. Thus, a silicon carbide substrate can form the basis for a “vertical” device with “top” and “bottom” ohmic contacts. In contrast, the insulating character of sapphire prevents its use in vertical devices.
In turn, n-type SiC substrates tend to be preferred over p-type substrates because n-type SiC is generally more conductive and transmits more light.
As a result, a Group III nitride device on a silicon carbide substrate typically includes an n-type substrate, an n-type buffer layer (or combination of layers), an n-type epitaxial layer, and a p-type contact layer (e.g., GaN) on the “top” of the device.
The development, commercial introduction, and use of such Group III nitride LEDs is relatively recent. Accordingly, it has been determined that in commercial use (the term “commercial” generally refers, but is not limited, to a product that is manufactured and sold on an inventory basis), they suffer from particular types of physical and chemical breakdown that eventually degrade the electronic performance of the devices. More specifically, it has become apparent that under normal environmental conditions, in which LED lamps are operated at or above room temperature and under normal conditions of humidity and other environmental factors, the epitaxial layers, ohmic contacts and associated passivation layers tend to interact with one another resulting in degraded optical and electrical performance. The degradation problem appears to be particularly acute in those devices that include p-type GaN as their top layer, with an ohmic contact to that p-type layer.
A particular form of degradation that is highly undesirable in LED lamps is an increase in forward voltage over time (VF degradation). “Forward voltage” refers to the voltage that must be applied across the terminals of an LED to cause it to emit light. VF degradation can lead to higher operating temperatures and increased power consumption over the life of the device.
Thus, in some commercial versions of blue LEDs made from Group III nitrides, the packaging itself is very specific and robust because the LED chip being packaged is relatively fragile even under normal environmental circumstances. For example, in the NSPG630S device from Nichia Chemical Industries of Tokushima, Japan, the p-type layer, the ohmic contact, and the passivation layer are coated with a flexible transparent polymeric material and then encapsulated in a hard resin such as an epoxy-based polymer.
For instance, in European Published Application No. 0 622 858 (“Gallium nitride based Ill–V group compound semiconductor device and method of producing the same”), Nakamura et al. report that, “(t)he p-electrode (to the p-type gallium nitride) may be formed of any suitable metallic material” (page 6, line 7). Nakamura goes on to list eight candidate metals (Au, Ni, Pt, Al, Sn, In, Cr, and Ti) and names a nickel and gold combination (page 6, lines 10–12 and 33–35) as the preferred selection. Furthermore, in selecting a passivation layer (“protective film”), Nakamura offers some merely general criteria (“The material forming the protective film is not particularly limited, as long as it is transparent, and electrically insulative.” Page 9, lines 31–32). Nakamura then goes on to list four candidate materials: silicon dioxide (SiO2), titanium oxide (TiO), aluminum oxide (Al2O3), and Silicon nitride (SiN).
The more widespread introduction of GaN-based LEDs has demonstrated, however, that such a general selection of materials is inappropriate, and that the resulting LEDs degrade much more rapidly than is otherwise appropriate for useful commercial devices. In particular, LEDs that: (1) include a top epitaxial layer of p-type GaN; (2) use ohmic contacts formed from certain metals (or their combinations) such as titanium and gold (“Ti/Au”); and (3) use silicon dioxide (SiO2) as the passivation layer, tend to exhibit more rapid degradation than is commercially acceptable. More specifically, it appears that the water-permeability of SiO2 allows sufficient moisture to reach the p-electrode to degrade the electrode and eventually the entire device relatively quickly.
As noted above, sophisticated packaging offers one option for protecting a relatively fragile die structure. In order to obtain their fullest commercial potential, however, blue LEDs formed from Group III nitrides must be manufactured in such a manner that they can be incorporated in more common lamp packages analogous to the lamp packages used for materials that are less esoteric than Group III nitrides.
Although the devices described in the '409 application demonstrated improved capabilities, some degradations problems persist.
Accordingly, a continuing need exists for a robust LED chip that can be packaged in normal fashion and yet which will successfully withstand both normal and elevated temperature and humidity conditions, for a time period sufficient to make the devices useful in a wide variety of commercial applications.